This invention relates to a film evaporator which may be used for obtaining highly viscous products by evaporating and removing solvents and other volatile components from viscous liquids such as high-polymer resins formed by polymerization.
A conventional agitated film evaporator is shown schematically in FIG. 1 of the accompanying drawings. This evaporator includes a vertical, generally cylindrical jacket 10 surrounding a sealed vessel 12 with a circumferential space 14 formed between them. The jacket has nozzles 16 and 18 for a heating fluid to flow through the space 14 and heat the peripheral wall of vessel 12. The inside of vessel 12 can be decompressed by connecting its vapor outlet nozzle 20 to a vacuum source via a condenser (not shown) outside the evaporator.
A vertical shaft 22 extends into the vessel 12 and is journalled by an upper sealed bearing 24 and a lower bearing 26, which is supported by a radial support 28 fixed to the vessel 12. The shaft 22 can be rotated by a motor (not shown) situated outside the evaporator. To the shaft 22 are fixed a distributor 30 having helical blades 31 and a film forming unit 32 having agitating blades 34 arranged under the distributor.
The vessel 12 can be supplied through an inlet nozzle 36 with a viscous liquid, which is dispersed by the distributor blades 31 over the inner surface of vessel 12. The descending liquid is then spread as a film on this surface by the agitating blades 34 in order to promote the evaporation of the volatile components. The vapor is discharged through the nozzle 20, while the processed liquid is discharged by a gear pump 38 through an outlet 40.
Generally, this type of film evaporator enhances evaporation with the static pressure of liquid being reduced by filming under decompressed condition. This can reduce the evaporation temperature and reduce the thermal influence on the liquid. The film is agitated, scraped and moved down by the agitating blades 34 while it is being formed. Consequently, highly viscous liquids can also be processed.
However, such conventional evaporators have certain disadvantages which are more noticeable when the liquid viscosity is higher. Thus, the processed highly viscous material descending on the inner vessel surface sticks to and collects on the lower bearing support 28 extending from this surface. If the accumulating material contacts the blades 34, the rotating system may lose its mechanical balance and rotation may be prevented.
Also, the collected material may change in quality under the prevailing thermal conditions during collection. If the changed material then becomes detached from the support 28 and mixes with the material being processed, the product quality is lowered. Further, below the film former 32 the highly viscous material descends only slowly under gravity to the pump 38. The material therefore remains within the vessel 12 for a long time, so that the material may deteriorate in quality.
The present invention seeks to provide a film evaporator which does not exhibit these difficulties when used with highly viscous liquids.